Slate is a metamorphic rock, formed under great pressure from what was originally mudstone. Its origins in Wales date back to the Paleozoic age between 350 and 500 million years ago. There are three types of slate found in Wales - Silurian, Ordovician and Cambrian.
Silurian is the most recent and is found in the Llangollen, Glyn Ceiriog and Corwen areas. It is not particularly durable but the industry in those parts developed because of its closeness to major markets. Its lack of durability is noticeable in the waste heaps which have turned back to shale in some cases.
Ordovician slate is found around Blaenau Ffestiniog and Corris, it is generally mined underground. It is almost uniformly blue-grey and is easily split using machines.
Cambrian is the oldest type and found in the north around Bethesda, Llanberis and in the Nantlle Valley. The colour is variable with a tendency towards purple, slates of this type are still mainly split and dressed by hand although there has been some success with mechanisation. (continued)
Pit quarries suffered from the disadvantage of requiring constant pumping, when work ceased the majority rapidly flooded leaving scenes like this at Dorothea Quarry, Nantlle.
A photo taken at Maenofferen quarry when it was still in production - this shows the tunnel or adit leading to an underground working. Also visible is a weighbridge, rubbish wagon, slab wagon and an upturned wagon turntable.
Six galleries can be seen in this photo, the galleries could be up to 75 feet high.
One thing that all slate quarries had in common was waste rock. On average 90% of the rock excavated ended up on the tips - in extreme cases it could be over 95%. These monumental tips are at Dinorwic Quarry.
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